This would indeed be a very good subject for a book; it would appeal to those interested in mines and those interested in railways which must be a HUGE target audience.

Underground rail haulage was used in a lot of the underground stone quarries (strictly not mines as they produced stone not minerals - that could open up another debate ! ! ) in the Bath area and in some of the Cotswolds area (for example Lower Balls Green). The gauge in the Bath area was generally 2 feet; there are a lot of rails still in situ in Monkton Farleigh (or Brown's Folly), including one area known as Clapham Junction for obvious reasons. There is a wealth of information about war usage of mines in Nick McCamley's excellent book 'Secret Underground Cities', which I am sure Mike Moore will sell you.

In Ireland the route of the main rail line from Dublin to Wexford goes inland between Wicklow and Arklow, not to avoid a hilly area, but purely because the railway already existed from Avoca down the valley to Arklow and a separate line up the valley from Avoca to Rathdrum, operated by separate rival mining companies on opposite sides of the valley; so existing mainline routes must be quite often determined by old mining railways.

Also in Ireland Shallee Mine, near Silvermines had a railway system in it. All the straight rails seem to have been taken out of the mine, probably in use somewhere nearby for fencing. However the interesting thing is that in the mine there are still several sets of points, crossovers and a turntable. I expect that if / when Shallee becomes a show mine the rail lines will be put back. Incidentally there is another main rail line (Limerick to Dublin) near to this mine.

At the lead & zinc Tara Mine at Navan, there was already a railway line which provided a direct link to Dublin port before the current mine started production. They really were lucky with their transport!

In Bristol, there was a rail network underground when the city sewage system was updated in the last 20 years or so and don't forget the Victorian Clifton Rocks Railway constructed entirely underground. More about these and railways in some Bristol coal mines is in Sally Watson's book 'Secret Underground Bristol'.

In the Isle of Man the Laxey Mines Research Group are using a railway in one of their digs in a coastal mine and cavers frequently use railways for cave digs too (eg Rhino Rift in Mendip).